For growing silicon single crystals, a CZ method has been used generally. The method includes bringing seed crystals into contact with the surface of a molten liquid of silicon contained in a crucible, rotating the crucible, and pulling-up the seed crystals upward while rotating the seed crystals in a direction opposite to a rotating direction of the crucible, thereby forming single crystals at a lower end of the seed crystals.
As shown in FIG. 10, in a related-art pulling-up method using the CZ method, a polysilicon is charged in a crucible 51 having a double structure of a quartz glass crucible 51a and a graphite crucible 51b, and the polysilicon is heated by a heater 52 to form a molten silicon liquid M. Then, seed crystals P attached to a pulling-up wire 50 are brought into contact with the molten silicon liquid M and silicon single crystals C are pulled-up.
Generally, before starting the pulling-up, necking of bringing the seed crystals P into contact with the molten silicon liquid M to melt a top end of the seed crystals P is performed after the temperature of the molten silicon liquid M is settled. The necking is an indispensable step for removing dislocation caused in silicon single crystals due to thermal shock generated upon contact between the seed crystals P and the molten silicon liquid M. By the necking, a neck portion P1 is formed as shown in FIG. 11.
For eliminating dislocation in the single crystals C formed below the neck portion P1, it is usually necessary that the neck portion P1 has a minimum diameter of 3 to 4 mm and a length of about 15 mm.
Further, as steps after starting the pulling-up, the following steps: a step for forming a crown portion of enlarging the diameter of crystals to a diameter of a straight trunk after the end of the necking; a step for forming the straight trunk of growing a constant diameter portion C2 after forming a shoulder C1; and a step for forming a tail portion of gradually decreasing the diameter of single crystals after the straight trunk forming step are performed.
The diameter of single crystals to be grown has been enlarged and the weight of single crystals to be pulled up has been increased in recent years. Accordingly, it is one of major subjects to prevent fracture of the neck portion P1 during pulling-up.
For the subject described above, Japanese Patent Application Publication No. JP-A-2008-189524 discloses a method of forming an enlarged diameter portion P1a and a reduced diameter portion P1b alternately to form a protuberant portion P1 so as to improve the strength of the neck portion P1 as shown in FIG. 12. According to the method, the strength of the neck portion P1 can be improved and the possibility of fracture can be decreased greatly than in a case of merely forming a narrow neck portion P1.
While the protuberant neck portion P1 described above has been formed so far by controlling the pulling-up speed of the seed crystals P and the temperature of the molten silicon liquid M, since the response characteristic of the temperature change for the molten silicon liquid M is poor, this is mainly depend on the control for the pulling-up speed of the seed crystals P.
Specifically, it is controlled by increasing the pulling-up speed of the seed crystal P in a case of intending to narrow the neck portion P1 and lowering the pulling-up speed of the seed crystal P in a case of intending to enlarge the neck portion P1.
It has been known that when the pulling-up speed of the seed crystals P is lower than a predetermined speed (for example, 2.5 mm/min or lower), a boundary M1 between the lower end of the neck portion P1 and the molten silicon liquid M has a concave shape as shown in FIG. 13A, whereas the boundary M1 has a convex shape when the pulling-up speed is increased to higher than the predetermined speed as shown in FIG. 13B.
However, when the pulling-up speed is increased in order to form a reduced diameter portion P1b by narrowing the neck portion P1, there is a subject that the boundary M1 has a convex shape as shown in FIG. 13B and dislocation tends to be caused when the boundary M1 has a convex shape.
In view of the subject described above, the applicant of the present invention has found that a narrow neck portion P1 (reduced diameter portion P1b) can be formed by lowering the number of rotation of a crucible even when the upper limit of the pulling-up speed of the seed crystals P is limited such that the boundary M1 has no convex shape.
For this purpose, control of limiting the pulling-up speed of the seed crystal P and lowering the number of rotation of the crucible when the upper limit is limited may be performed manually, but this results in a subject of requiring skilled workers, increasing the cost, and making the quality instable.